1. Field of the Invention
The present invention relates to a physical quantity measuring apparatus for measuring a physical quantity such as an acceleration applied to a movable body by detecting a position of the movable body, and controlling the movable body at a predetermined position on the basis of the result of the position detection.
2. Description of Related Art
As such a physical quantity measuring apparatus, there is known an electrostatically levitated gyroscope configured to detect a position of a movable body disposed in a casing so as to be able to be electrostatically levitated, and feedback-control the movable body at a predetermined position in order to measure the acceleration or angular velocity of the movable body. For more details, refer to Japanese Patent Application Laid-open No. 2005-140709, for example.
This electrostatically levitated gyroscope includes a plurality of electrostatic supporting electrodes disposed so as to face each other across the movable body in each of three axis directions orthogonal to one another, and common electrodes for taking in signals from the main body. To measure physical quantities (accelerations, angular velocities, for example), the electrostatic supporting electrodes are applied with respective position-controlling voltages to position the movable body at a predetermined position in the casing, and also applied with respective position-detecting carrier voltages having different frequencies.
The position-detecting carrier voltages are also applied to the movable body through capacitive coupling between the movable body and each of the electrostatic supporting electrodes. The amplitudes of the position-detecting carrier voltages applied to the movable body vary in accordance with the movement of the movable body in the casing.
Theses carrier voltages applied to the movable body as signals for detecting the position of the movable body are taken in through the common electrodes, and are subjected to synchronous detection to detect the displacements of the movable body with respect to each of the electrostatic supporting electrodes. The position-controlling voltages applied to the electrostatic supporting electrodes are adjusted in accordance with the detected displacements to keep the movable body at the predetermined position.
Based on the detected displacements of the movable body with respect to the electrostatic supporting electrodes, the accelerations or angular velocities applied to the movable body (or the casing) are measured.
In the above electrostatically levitated gyroscope, the-position detecting carrier voltages are superimposed on the position-controlling voltages to enable keeping the movable body at the predetermined position while detecting the position of the movable body.
If PWM signals are used as the position-controlling voltages, since noise occurring when each PWM signal rises or falls is superimposed to the position-detecting carrier voltages, the position detection of the movable body may be adversely affected.
To solve this problem, in the electrostatically levitated gyroscope disclosed in the Japanese Patent Application Laid-open No. 2005-140709, a triangular wave voltage is used as the position-detecting carrier voltages. In more detail, the triangular wave voltage is applied to the electrostatic supporting electrodes in a time-divisional manner such that the pulse ends of the PWM signals coincide with the turnaround points of the triangular wave voltage to prevent the noise occurring when each PWM signal rises or falls from adversely affecting the position detection of the movable body.
To increase precision of the PWM signals to thereby increase the accuracy of the position control of the movable body, it is necessary to increase also the frequency of the position-detecting carrier voltage (the triangular wave) so that the turnaround points of the position-detecting carrier voltage can accurately coincide with the pulse ends of the PWM signals.
However, increasing the frequency of the position-detecting carrier voltage leads to increase of total cost of the detection circuits included in the electrostatically levitated gyroscope. In addition, there is a limit in increasing the frequency of the position-detecting carrier voltage because of parasitic capacitance etc. Hence, it has been difficult to increase the accuracy of the position detection of the movable body by increasing the resolution of the PWM signal.